1. Field
Aspects of embodiments according to the present invention are directed toward the field of power amplifiers, and in particular, performance optimization of power amplifiers in RF systems.
2. Description of Related Art
In an RF system, e.g., a radar system, wireless communication system, etc., power amplifiers are designed to efficiently convert DC power to RF power. To achieve this goal effectively, the constituent transistors are presented with favorable impedances that optimally perform this energy conversion. In addition, the network that presents this favorable impedance also provides an efficient (low loss) power transformation to the follow on circuitry. It is desirable to maintain this low loss transformation, however this results in the performance of the power amplifier being highly sensitive to the load (e.g., impedance) presented by the follow on circuitry. A mismatch between the power amplifier and the load presented to it may in turn enhance or deteriorate DC-to-RF power conversion. Poor power conversion at the output stage results in power dissipated within the transistor of the output stage and correspondingly an increase in the device's temperature (e.g., junction temperature for a bipolar transistor or a field effect transistor) and a corresponding reduction in long-term reliability. Therefore, an optimized balance between RF power generation and long term reliability is desired.
In the related art, a circulator/isolator is typically utilized to protect the power amplifier from mismatch conditions. The circulator/isolator is used to control the impedance seen by the output stage of the power amplifier. However, the bandwidth of the circulator/isolator may limit utilization of the available area for the aperture. For example, in an active electronically scanned antenna (AESA), the circulator/isolator provides the duplexing function between transmit and receive, as well as to isolate the power amplifier from the active impedance of the AESA. While the circulator/isolator can mitigate the mismatch problem between the power amplifier and the load, as the operational bandwidth increases and/or the low end frequency gets lower, the size and weight of the circulator/isolator becomes a limiter in terms of how many active elements can be provided given space constraints. Some AESA designs partition the active area into a transmit and a receive aperture because the bandwidth exceeds that possible with typical circulator/isolator technology. However, such an approach reduces the available area for use for both the transmit and receive functions and does not alter the large impedance variation presented to the power amplifier as the beam is steered.